The degradation of organic matter is a process that occurs aerobically and/or anaerobically in nature by the complex interaction of a multitude of microorganisms. Controlled more or less industrialized methods have been known for a long time and referred to as composting for aerobic processes and fermentation, bio-methanisation, or simply anaerobic digestion (AD), for the anaerobic processes. AD processes may be divided into two principle steps: hydrolysis and methanogenesis.
The two processes of composting and AD have been developing independently over the last many years. Composting as a way to produce compost for fertilization and soil improvement has been thoroughly described many places. Likewise, one-stage, two-stage, or multi-stage AD methods have been developed on the basis of the most varied developments. In addition to wet AD developed from liquid-manure and wastewater treatment, dry AD is also practiced.
The principle of the two-step, two-stage AD was described for the first time by Ghosh in 1978. In this process, wastes are percolated in an anaerobic reactor. The percolation water is subsequently fermented to biogas in a methane reactor. The method was further developed and patented by Rijkens and Hofank (U.S. Pat. No. 4,400,195) in the 1980s for organic wastes. The method was converted twice to practice, to the ANM method in Ganderkesee and to the Prethane-Rudad method in Breda. Tests were conducted by Wellinger and Suter in the 1980s with solid manure, and by Widmer with market and meatpacking wastes. In the latter case, the percolator was also driven in an aerobic environment.
The newest plants, which have been developed according to this method (for residual wastes) are the ISKA® percolation method in Sansenheck, and the BIOPERCOLAT® method (DE 198 46336 A1). Here, the waste is hydrolyzed in a percolator after a mechanical pretreatment (for example, sieving, metal separation). The percolator is therefore equipped with a stirring mechanism, so that the wastes are continually transported through the reactor. After a residence time of 2 to 3 days, the percolate is discharged free of water and ready for further treatment or depositing in a layer. The percolation water is fermented anaerobically to biogas in a methane reactor after separation of sand and fibers. The water that is cleaned in this way is directly used as percolation water or is used after another cleaning (e.g., for removing nitrogen).
Large amounts of organic solid wastes are produced in many towns and cities all over the world, and treatment methods are often limited by solids handling requirements for adequate sterilization of these large amounts of heavy, solid waste. Accordingly, there is a need for improved systems and methods for digestion and composting of solid wastes with limited solids handling requirements.